1. Field of the Invention
This invention relates to an idling speed control system for stabilizing revolution of the engine during idling.
2. Description of the Prior Art
There has been known an idling speed control system in which the engine speed during idling is caused to converge on a target idling speed by feedback control of the ignition timing in which the ignition timing is controlled according to the difference between the actual engine speed during idling and the target idling speed, as disclosed for instance in Japanese Unexamined Patent Publication No. 56(1981)-121843.
A control variable for the feedback control of the ignition timing in order to cause the engine speed during idling to converge on the target idling speed such as a reference ignition timing, a range of control, a feedback control gain or the like has been determined so that it conforms to the state of the engine where the engine idles under no external load.
However, when the engine idles under external load such as an air conditioner, air is fed to the engine in a larger amount than when the engine idles under no load in order to keep the engine speed at a value equal to that when the engine idles under no load. When the amount of intake air increases, the burning rate increases, and accordingly, when the engine idles under no load, the engine output torque is maximized at an earlier ignition timing than when the engine idles under load as shown in FIG. 7. Further as can be seen from FIG. 7, the change in the engine output torque with change in the ignition timing is larger when the engine idles under load than when the engine idles under no load. Accordingly, when the feedback control of the ignition timing is effected during idling under load on the basis of the reference ignition timing for idling under no load, the engine output torque can be lowered since the reference ignition timing is too early to obtain the maximum engine output torque during idling under load. Further when the feedback control of the ignition timing is effected during idling under external load on the basis of the control gain for idling under no load, the engine output torque changes by a larger amount for a given change of the ignition timing than during idling under no load, whereby revolution of the engine cannot be stabilized.